ChirpChart Shake Plugin

The Chirpchart plugin draws a chirp chart test image.

A chirp chart is a sine wave which is increases it frequency
towards the edge of the image. It is used to examine how
cameras, digitisers, digital processing and monitors respond
to different spatial frequencies.

When using a chirp chart, you are looking for two effects -
Attenuation - where the sine wave stops looking black
and white and starts looking grey - and aliassing or
moiré patterns appearing. These look like phantom
circles - usually grey - appearing where they
shouldn't.

The above chart has aliasing: all the circles should spread out
from the bottom left corner. The ones from the other
corners and the circles near the centre of the image are
phantom circles caused by aliassing.

Always view a chirp chart at 1:1 resolution - you'll see in a
moment why.

Experiments with a chirp chart

Monitoring your monitor

Create a chirp chart, and set it to be as big as possible. Fill the
screen with it (push the space bar to make the viewer the
only thing in Shake's window). Keep the resolution at 1:1
(push Home, just to be sure). Now stand back. Squint at
it. Does it like equally white all over, or do some of the
circles look greyer? Is there a greyish band on the image?
Maybe the white parts are consistent, but the blacks seem to
be brighter in parts. If you see this, it's because the
monitor cannot display certain frequencies.
Avoid using a monitor which exhibits grey bands for detailed finishing
work.

Now look at the colours. Is it white all over, or are their
pinky/bluey bits? This is the same problem, the monitor
cannot display certain frequencies, but it's abilities
change depending on the colour. Again, it's a sign of a bad
monitor.

Monitoring your effects

Look at your chirp chart zoomed out to a 1:2 view. You will immediately see phantom
circles appearing in the image. Shake uses a fast and crude
scaling technique in the viewer. In a real image you'd see
jaggies, on a chirp chart you see phantom circles.

Zoom back to 1:1 and add a Zoom node to the Chirp Chart. Set its
scaling to 0.5 The cirlces reappear, but not as badly as
before. Shake is doing a better job of scaling the image.
Try a few different filter types. Notice that some
(like Dirac) have a lot of aliassing, a lot of fake circles,
some (like triangle) attenuate the image, so the stripes in
the top right corner look a lot darker than the ones in the
bottom left.

Replace the Zoom node with a blur node. Increase the amount of blur
and watch what happens. Perhaps it's a bit surprising: the
image doesn't actually get any more blurry, just the top
right hand part of the image goes grey. A blur filter is
a low-pass filter - it takes away high frequency
detail and leaves low frequency information intact. Since in
the chirp chart, all the high frequencies are in the top
right hand corner, that part of the image disappears. The
blur has also aliassed - those phantom circles are back.

Now try a rotate of 2 degrees isntead of the blur. You'll see a patchwork of greyer
values. The rotate operator seems to attenuate, but it
doesn't seem to alias. That's good news.

Chirp Charts are useful for seeing what effects are doing to your
images - you can detect whether they will loose high
frequency detail by looking for where the top right hand
corner of the image goes grey, and you can look for where they'll
introduce jaggies by looking for phantom circles - the
closer the circles appear to the bottom left hand corner of
the image, the worse the jaggies will look.

Monitoring your camera

Render and print out a chirp chart. Check closely for aliassing on
the print-out.
(yes, printers can introduce problems too). Now film it with
the camera. Move the chart further and further away from the
camera. What happens when parts of the chart become too
finely detailed to capture clearly? Do they attenuate
and become grey, or do those phantom circles appear - does
your camera alias? If it aliasses, what colour are
the phantom circles? Most single CCD cameras will alias in
different colours at different times, usally the red
and blue and circles appear closer to the bottom left corner
than the green ones. If your phantom circles are white,
that's good news - it means you won't get weird coloured jaggies and
other bizarre effects in your finished shot.

This is an extremely useful
procedure to go through before purchasing or hiring a
camera: it will show you exactly how much detail the camera
can catch before it attenuates or aliasses, and it will
indicate how this will happen, and whether it will mess up
the colours when it does. The Resolving Power of
a camera is found by measuring how close the lines can get
before attenuation or aliassing become noticeable.